The present invention relates to a process for producing alcohols obtainable by hydrogenation of aldehydes. Particularly, it relates to a process for producing three types of alcohols i.e. a straight-chain alcohol and a branched chain alcohol each having a carbon number of n (where n=4 to 6) and a branched chain alcohol having a carbon number of 2n, in parallel, from a straight-chain and branched chain mixed aldehyde product having a carbon number of n, formed by hydroformylation of an olefin, whereby the energy cost and the installation cost can be reduced.
A process wherein a mixed aldehyde product is obtained by a so-called hydroformylation reaction i.e. by reacting an olefinic unsaturated compound with carbon monoxide and hydrogen in the presence of a complex catalyst having a Group VIII metal such as rhodium modified with a ligand containing phosphorus, the aldehyde product is separated by distillation into a straight-chain aldehyde and a branched chain aldehyde, the straight-chain aldehyde is subjected to aldol condensation to obtain a dimer of the straight-chain aldehyde, and the dimer is hydrogenated to obtain an alcohol having a carbon number of 2n, a process wherein the separated straight-chain aldehyde is directly hydrogenated to obtain a straight chain alcohol having a carbon number of n, and a process wherein the separated branched chain aldehyde is hydrogenated to obtain a branched chain alcohol having a carbon number of n, have been worldwidely industrially practiced. The above-mentioned mixed aldehyde product is one remaining after separating from the hydroformylation reaction zone the majority of an unreacted olefin, the solvent and the catalyst solution containing high boiling point by-products, and it contains, in addition to the straight-chain and branched chain aldehydes as the main components, a very small amount of dissolved gas (such as hydrogen, carbon monoxide, methane or carbon dioxide), a small amount of an unreacted olefin, paraffins and water, which are lighter than the main component aldehydes, an aldehyde having a carbon number smaller by one than the main component aldehydes, and a small amount of the solvent and high boiling point by-products, which have higher boiling points than the main component aldehydes.
Accordingly, a product obtained by separating the catalyst solution from the mixed aldehyde product is an isomer mixture comprising mainly a straight-chain aldehyde and a branched chain aldehyde. However, it is rare that this isomer mixture is used as it is, as a reaction material for the subsequent step, and it is commercially common to purify and separate the mixture into the straight-chain aldehyde and the branched chain aldehyde before use. As disclosed in U.S. Pat. No. 5,227,544, for example, in a case where 2-ethylhexanol having a carbon number of 2n is produced by aldol condensation of n-butyraldehyde, followed by hydrogenation, isobutyraldehyde contained as an impurity in said n-butyraldehyde is likely to finally change into isooctanol, thereby deteriorating the quality of 2-ethylhexanol product substantially. The above purification and separation is intended to prevent such deterioration of the product. On the other hand, with respect to isobutyraldehyde, for example, in a case where isobutyraldehyde is hydrogenated to obtain isobutanol, n-butyraldehyde contained as an impurity in said isobutyraldehyde, is likely to deteriorate the quality of the isobutanol product substantially, or substantially increase the cost for distillation for purification to maintain the quality. The above purification and separation is intended to avoid such drawbacks.
With respect to a conventional technique for separation of aldehyde isomers, for example, JP-A-4-273841 discloses a method wherein a mixed aldehyde product is distilled in a single distillation column to simultaneously obtain three types of independent product streams i.e. a purified branched chain aldehyde stream and two types of purified different straight chain aldehyde streams.
Further, U.S. Pat. No. 5,227,544 discloses a method wherein a small amount of water is added to a distillation column for aldehyde to hydrolyze an oligomer of isobutyraldehyde contained in crude butyraldehyde and to distill the entire amount as a monomer, thereby to produce 2-ethylhexanol of high purity.
Further, JP-A-8-169858 discloses a method of obtaining a straight-chain aldehyde from the bottom and a branched chain aldehyde from a position within theoretical plate numbers of from 5 to 15 plates from the top and above the feeding position, in the step for separating the mixed aldehyde product.
Further, JP-A-8-208554 discloses a method wherein a mixed aldehyde product is separated under a column top pressure of from 0.001 to 0.5 kg/cm2G and a column bottom pressure of from 0.05 to 1.0 kg/cm2G, whereby n-butyraldehyde and isobutyraldehyde can be separated with a reduced energy cost.
The above-described methods are all concerned with a method for separating a mixed aldehyde. However, even by means of these methods, the aldehyde distillation column for separating the straight-chain aldehyde and the branched chain aldehyde, requires a large plate number and a large reflux amount, since the boiling points of aldehyde isomers to be separated, are very close to one another. Therefore, in order to obtain purified alcohols from such a mixed aldehyde economically, it is necessary to suppress the energy cost and the installation cost of the aldehyde distillation column.
Heretofore, industrially, improvement of the energy cost has been attempted by changing of the operation conditions or recovering waste heat from other heat generating step in the process. However, a method for more economically producing purified alcohols from a mixed aldehyde product, has been desired.
Accordingly, it is an object of the present invention to provide a process for economically producing alcohols from a mixed aldehyde product by reducing the energy cost and the installation cost of the aldehyde distillation column.
The present inventors have conducted an extensive study on the above subject and as a result, have found it possible to substantially reduce the energy cost such as a heat load and the theoretical plate number as compared with separating the mixed aldehyde in an aldehyde distillation column, by obtaining from the top of the aldehyde distillation column a fraction containing at least 30 wt %, preferably at least 50 wt %, of an aldehyde having a higher boiling point and separating a mixed alcohol obtained by hydrogenation of a mixed aldehyde obtained from the top of the column, into a straight-chain alcohol and a branched chain alcohol. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides a process for producing alcohols, to obtain a straight-chain alcohol having a carbon number of n (where n=4 to 6), a branched chain alcohol having a carbon number of n (where n=4 to 6) and a branched chain alcohol having a carbon number of 2n (where n=4 to 6) from a mixed aldehyde comprising a straight-chain aldehyde having a carbon number of n (where n=4 to 6) and a branched chain aldehyde having a carbon number of n (where n=4 to 6) in an optional proportion, which comprises supplying the mixed aldehyde to a distillation column, withdrawing from the bottom of the column an aldehyde rich in the straight-chain aldehyde, dimerizing the straight-chain aldehyde, followed by hydrogenation to obtain the branched chain alcohol having a carbon number of 2n, while obtaining, as a fraction from the top of the column, an aldehyde rich in the branched chain aldehyde and having a straight-chain aldehyde concentration in the fraction of at least 30 wt %, preferably in the fraction of at least 50 wt %, subjecting the fraction to hydrogenation, and purifying and separating the resulting straight-chain and branched chain mixed alcohol to obtain the straight-chain alcohol having a carbon number of n and the branched chain alcohol having a carbon number of n, respectively.
Now, the present invention will be described in detail.
In the present invention, the mixed aldehyde to be separated and purified, is an aldehyde mixture comprising mainly a straight-chain aldehyde-having a carbon number of from 4 to 6 and a branched chain aldehyde having a carbon number of from 4 to 6. Such a mixed aldehyde is not particularly limited. However, it is preferred to employ, for example, the above-mentioned mixture obtained by hydroformylation of an olefin having a carbon number of from 3 to 5 such as propylene or butene.
An example of the method for producing a mixed aldehyde by a hydroformylation reaction will be described. The starting material propylene or butene may usually be employed without any special pre-treatment. However, a sulfur content or a halogen content known to be a catalyst poison, a diene, a triene, a peroxide, etc., may be removed by a conventional method such as adsorption, extraction, distillation, heat treatment or separation by a membrane, before use.
As the catalyst, a rhodium catalyst containing an organic phosphorus compound as a ligand, can be used. The organic phosphorus compound may, for example, be a trialkylphosphine such as tributylphosphine or trioctylphosphine, a triarylphosphine such as triphenylphosphine, tritolylphosphine or a triarylphosphine having hydrogen of a phenyl group substituted by e.g. a sulfonic group or a halogen, a tricycloalkylphosphine such as tricyclohexylphosphine, an alkylarylphosphine such as monobutyldiphenylphosphine or dipropylphenylphosphine, a cycloalkylarylphosphine, or an alkylcycloalkylphosphine. Further, a trialkylphosphite, a triarylphosphite such as triphenylphosphite or trinaphthylphosphite, which may have a substituent, or an alkylarylphosphite, may also be employed. Specifically, compounds disclosed in U.S. Pat. Nos. 3,415,906, 4,599,206, 4,351,759, 4,748,261, 4,567,306, 5,235,113 and 5,227,532 may be mentioned. However, in the present invention, the type of such an organic phosphorus compound is not limited.
Further, two or more among these organic phosphorus compounds may be used as a mixed ligand. Further, such an organic phosphorus compound may be used in admixture with a pentavalent organic phosphorus compound such as triphenylphosphine oxide.
As the rhodium source, in addition to a rhodium complex such as rhodium
hydridocarbonyltris(triphenylphosphine) or rhodium acetoxybis(triphenylphosphine), an oxide such as rhodium acetylacetonate or rhodium acetate may, for example, be used. The rhodium source may directly be supplied to the hydroformylation reactor, but may preliminarily be treated with carbon monoxide and hydrogen in a solvent at a high temperature and pressure together with the ligand of an organic phosphorus compound outside the reactor, to prepare a catalyst solution. The solvent for the preparation of this catalyst solution is selected usually among the solvents for reaction as described hereinafter, but may not be the same as the solvent for reaction. With respect to the conditions for preparation of the catalyst, the rhodium concentration is usually from a few ppm to a few wt %, the ratio of the ligand of an organic phosphorus compound to rhodium is usually P/Rh=1 to 10,000 in a molar ratio, the temperature is from 60 to 200xc2x0 C., the pressure is from atmospheric pressure to 200 kg/cm2G, and the treating time is within a range of from a few minutes to a few tens hours.
The above treatment may be carried out by a batch system or a continuous system.
As the solvent for the hydroformylation reaction, the olefin itself may be used as the solvent, or a formed aldehyde or a high boiling point substance produced as a by-product by the reaction, may be used. Further, various solvents which are capable of dissolving the catalyst and which give no adverse effects to the reaction, may be employed, such as an aliphatic hydrocarbon such as hexane or octane, an aromatic hydrocarbon such as toluene or xylene, an alcohol such as butanol, 2-ethylhexanol, ethylene glycol or propylene glycol, an ether such as triglyme, an ester such as dioctyl phthalate, or water.
The hydroformylation reaction may be carried out usually under such conditions as a hydrogen partial pressure of from 0.1 to 200 kg/cm2, a carbon monoxide partial pressure of from 0.1 to 200 kg/cm2, a total pressure of a few kg/cm2 to 300 kg/cm2, a ratio of hydrogen partial pressure/carbon monoxide=0.1 to 10, a temperature of from 60 to 200xc2x0 C., a rhodium concentration of from a few ppm to a few wt %, a molar ratio of P in the organic phosphorus compound ligand/Rh=1 to 10,000, and a reaction time of from a few minutes to a few tens hours.
The method for obtaining the mixed aldehyde from the reaction zone of hydroformylation which is carried out as described above, is not particularly limited and may, for example, be a method by gas stripping as disclosed in JP-A-52-125103 or a method by distillation as disclosed in JP-A-54-89974. By adopting any means, as a result, the majority of an unreacted olefin, the solvent and the catalyst solution containing high boiling point by-products, will be removed, and the components containing in the mixed aldehyde will be, other than the main component mixed aldehyde, a very small amount of dissolved gas (such as hydrogen, carbon monoxide, methane or carbon dioxide), a small amount of an unreacted olefin, paraffins and water, which are lighter than the aldehydes, a fraction lighter than the main component aldehydes, such as a low boiling point aldehyde having a carbon number smaller by one than the main component aldehydes, and a small amount of the solvent and high boiling point by-products, which have boiling points higher than the main component aldehydes.